BOSTON — Documents in a lawsuit against United Airlines, which claim negligence in security leading to the Sept. 11, 2001 attacks, show that a United flight attendant spotted one of the hijackers on a Boston-bound flight 12 days before the attacks.

Flight attendant Greg McAleer was working United Flight 514 from Chicago to Boston on Aug. 20, 2001. McAleer testified to investigators from the 9/11 Commission that as he was greeting passengers a man in casual clothes displayed a jump-seat pass.

United issued such a pass to pilots from other airlines. It allowed them to ride as passengers in the jump seat in the cockpit. The flight’s pilot would not let the passenger sit in the jump seat because he did not have proper ID, but he was allowed to fly as a passenger.

After Sept. 11, McAleer recognized the man as Fayez Banihammad, one of the 19 hijackers.

Attorneys for the family of Mark Bavis, one of the victims aboard United Flight 175 out of Logan International Airport, are trying to depose federal agents who interviewed the flight attendant, but the Department of Justice is refusing to let the agents testify.